The preparation of high molecular weight polytetramethyleneadipamide is known from e.g. EP-B-No. 0039524, corresponding to U.S. Pat. No. 4,408,036, and 0038094, corresponding to U.S. Pat. No. 4,460,762, in which the process is carried out in two steps, viz. a precondensation reaction between 1,4-diaminobutane and adipic acid, resulting in a prepolymer with a number-average molecular weight of less than approx. 10,000, which is subsequently after-condensed in the solid phase. Before the prepolymer is subjected to the after-condensation reaction, it is ground to particles with an average diameter ranging from 0.1 to 0.5 mm. The after-condensation is carried out in a tumbling dryer or other type of reactor in which the particles are continuously in motion.
In practice, an economical large-scale application of such a method of after-condensation and conversion of the method into a continuous process is difficult to realize. When using a tumbling dryer, fouling of the wall, which may lead to lump formation and a colored product, occurs frequently as a result of the sintering and caking of the after-condensing particles. Furthermore, the high molecular weight product resulting from the after-condensation must be melted and extruded into pellets, in which form it is suitable for commercial sale
Processes carried out in the melt offer certain advantages in this respect. They are described in EP-A-No. 0160337. A disadvantage is that, because the melting point of polytetramethyleneadipamide is very close to its decomposition temperature, the temperatures have to be controlled very accurately, and the residence times must be restricted. The result of the latter is that it becomes very difficult to obtain a high molecular weight product in a single process. This subsequent after-condensation in the solid phase must be carried out. This after-condensation is preferably carried out with a polymer consisting of particles of a defined size, obtained, e.g., by melt extrusion, in order to avoid case the previously mentioned disadvantages of the after-condensation of small particles. This is a rather time-consuming procedure, however, and the first part of the after-condensation process in the melt is sensitive to interference.
An object of the present invention is therefore to provide polyamide pellets for use in the production of high molecular weight polytetramethyleneadipamide articles which avoid these disadvantages and facilitate carrying out a continuous process on a technical scale. "High molecular weight polytetramethyleneadipamide" refers to a polyamide that can readily be used for further processing, for instance by spinning, extrusion and molding into end products. In general its .eta..sub.rel should be at least 2.0.
A further object of the invention is to provide the prepolymer obtained from the first condensation reaction in a form which is suitable for after-condensation in the solid phase in a single continuous process, requiring no additional processing steps, such as melt extrusion, to render the high molecular weight product suitable for further processing.
A still further object of the invention is to provide after-condensed high molecular weight polyamide in a form suitable for further processing.